Metal-nonmetal transition in LixCoO2 thin film and thermopower enhancement at high Li concentration

We investigate the transport properties of LixCoO2 thin films whose resistivities are nearly an order of magnitude lower than those of the bulk polycrystals. A metal-nonmetal transition occurs at ~0.8 in a biphasic domain, and the Seebeck coefficient (S) is drastically increased at ~140 K (= T*) with increasing the Li concentration to show a peak of magnitude ~120 \muV/K in the S-T curve of x = 0.87. We show that T* corresponds to a crossover temperature in the conduction, most likely reflecting the correlation-induced temperature dependence in the low-energy excitations

Dual electronic states in thermoelectric cobalt oxide

We investigate the low temperature magnetic field dependence of the resistivity in the thermoelectric misfit cobalt oxide [Bi1.7Ca2O4]0.59CoO2 from 60 K down to 3 K. The scaling of the negative magnetoresistance demonstrates a spin dependent transport mechanism due to a strong Hund's coupling. The inferred microscopic description implies dual electronic states which explain the coexistence between localized and itinerant electrons both contributing to the thermopower. By shedding a new light on the electronic states which lead to a high thermopower, this result likely provides a new potential way to optimize the thermoelectric properties

Interface and electronic characterization of thin epitaxial Co3O4 films

The interface and electronic structure of thin (~20-74 nm) Co3O4(110) epitaxial films grown by oxygen-assisted molecular beam epitaxy on MgAl2O4(110) single crystal substrates have been investigated by means of real and reciprocal space techniques. As-grown film surfaces are found to be relatively disordered and exhibit an oblique low energy electron diffraction (LEED) pattern associated with the O-rich CoO2 bulk termination of the (110) surface. Interface and bulk film structure are found to improve significantly with post-growth annealing at 820 K in air and display sharp rectangular LEED patterns, suggesting a surface stoichiometry of the alternative Co2O2 bulk termination of the (110) surface. Non-contact atomic force microscopy demonstrates the presence of wide terraces separated by atomic steps in the annealed films that are not present in the as-grown structures; the step height of ~ 2.7 A corresponds to two atomic layers and confirms a single termination for the annealed films, consistent with the LEED results. A model of the (1 * 1) surfaces that allows for compensation of the polar surfaces is presented.Comment: 8 pages, 7 figure

Thermoelectric power factor under strain-induced band-alignment in the half-Heuslers NbCoSn and TiCoSb

Band convergence is an effective strategy to improve the thermoelectric performance of complex bandstructure thermoelectric materials. Half-Heuslers are good candidates for band convergence studies because they have multiple bands near the valence bad edge that can be converged through various band engineering approaches providing power factor improvement opportunities. Theoretical calculations to identify the outcome of band convergence employ various approximations for the carrier scattering relaxation times (the most common being the constant relaxation time approximation) due to the high computational complexity involved in extracting them accurately. Here, we compare the outcome of strain-induced band convergence under two such scattering scenarios: i) the most commonly used constant relaxation time approximation and ii) energy dependent inter- and intra-valley scattering considerations for the half-Heuslers NbCoSn and TiCoSb. We show that the outcome of band convergence on the power factor depends on the carrier scattering assumptions, as well as the temperature. For both materials examined, band convergence improves the power factor. For NbCoSn, however, band convergence becomes more beneficial as temperature increases, under both scattering relaxation time assumptions. In the case of TiCoSb, on the other hand, constant relaxation time considerations also indicate that the relative power factor improvement increases with temperature, but under the energy dependent scattering time considerations, the relative improvement weakens with temperature. This indicates that the scattering details need to be accurately considered in band convergence studies to predict more accurate trends.Comment: 21 pages, 8 figures. arXiv admin note: text overlap with arXiv:1905.0795

Large-scale Synthesis of β-SiC Nanochains and Their Raman/Photoluminescence Properties

Although the SiC/SiO2 nanochain heterojunction has been synthesized, the chained homogeneous nanostructure of SiC has not been reported before. Herein, the novel β-SiC nanochains are synthesized assisted by the AAO template. The characterized results demonstrate that the nanostructures are constructed by spheres of 25–30 nm and conjoint wires of 15–20 nm in diameters. Raman and photoluminescence measurements are used to explore the unique optical properties. A speed-alternating vapor–solid (SA-VS) growth mechanism is proposed to interpret the formation of this typical nanochains. The achieved nanochains enrich the species of one-dimensional (1D) nanostructures and may hold great potential applications in nanotechnology

Biomimetic Micropatteming of Titanium Dioxide Thin Films for Gate Dielectrics

We have succeeded in fabricating micropatterns of TiO2 thin films on self-assembled monolayers (SAMs). SAMs of OTS (octadecyltrichlorosilane) were formed on Si wafers and modified by UV irradiation using a photomask to generate octadecyl/silanol-pattern. They were used as templates to deposit TiO2 thin films by the use of TDD (titanium dichloride diethoxide) dissolved in toluene. Amorphous films were selectively deposited on silanol regions. Line width variation of the pattern was improved to be well below the current electronics design rule, 5%. Dielectric constant of an as-deposited TiO2 thin film, dielectric properties of TiO2/SiO2/Si interface and leakage current density were evaluated by measuring I-V and C-V characteristics of the MOS (metal-oxide-semiconductor) device. Rather low leakage current was observed under high electric fields and the TiO2 thin film with the thickness ～18 nm showed dielectric constant of ～22 at 100 kHz, which is more than 5 times as large as that of a usual SiO2 dielectric layer. However, the resistivity of the TiO2 film was estimated to be not high enough and the dielectric constant depended on frequency, both of which are disadvantages for a gate dielectric

Biomimetic Micropatteming of Titanium Dioxide Thin Films for Gate Dielectrics

We have succeeded in fabricating micropatterns of TiO2 thin films on self-assembled monolayers (SAMs). SAMs of OTS (octadecyltrichlorosilane) were formed on Si wafers and modified by UV irradiation using a photomask to generate octadecyl/silanol-pattern. They were used as templates to deposit TiO2 thin films by the use of TDD (titanium dichloride diethoxide) dissolved in toluene. Amorphous films were selectively deposited on silanol regions. Line width variation of the pattern was improved to be well below the current electronics design rule, 5%. Dielectric constant of an as-deposited TiO2 thin film, dielectric properties of TiO2/SiO2/Si interface and leakage current density were evaluated by measuring I-V and C-V characteristics of the MOS (metal-oxide-semiconductor) device. Rather low leakage current was observed under high electric fields and the TiO2 thin film with the thickness ～18 nm showed dielectric constant of ～22 at 100 kHz, which is more than 5 times as large as that of a usual SiO2 dielectric layer. However, the resistivity of the TiO2 film was estimated to be not high enough and the dielectric constant depended on frequency, both of which are disadvantages for a gate dielectric